Currently, during a pull-request it is not possible to change the destination branch to be a different branch than the issuer specified.
It is possible to create a new branch on pull with the same name as the source branch, but this must be done on the issuer side. If the issuer is proposing a merge into master, I don't necessarily want the code to be merged into master without running my own test suites.
The request can be rejected and then asked to change the destination branch, but this is clunky. I could pull directly from their repository, into my local copy before pushing to my online repository but this makes it more difficult to perform larger peer reviews --> telling everyone to pull from that branch instead of just pulling the latest copy from the blessed repository is not intuitive. I also cannot do this if their fork is private.
I propose the owner of the destination repository should be able to change the destination branch before pulling into his repository from the pull-request UI.
This is how I work locally when interacting with colleagues in my office, so being able to work like this from Bitbucket would be very convenient.